This invention relates generally to an apparatus for damping perturbations in pulp suspension flow and, more particularly, to an apparatus for damping pressure and consistency perturbations in the pulp suspension flow to the headbox of a paper making machine.
Conventional paper making machines typically include a headbox which receives a flow of a pulp suspension and delivers the same to the web former section of the machine. Of course, it is desired that the quantity of dry matter in the pulp system issued from the aperture of the headbox be uniform over the length of the aperture as well as uniform per unit of time.
Thus, if the flow rate is uniform over the length of the headbox aperture but varies with respect to time, this will result in variation in the dry weight of the paper manufactured in the direction in which the web is formed in the machine. Such variations in the dry weight content of the pulp per unit time may be caused by the variations of the volume flow rate of the pulp to the headbox, by the pressure waves which are propagated at the velocity of sound and which are always present in the tube which feeds to the headbox, which pressure waves manifest themselves as variations in the kinetic energy of the pulp emanating from the headbox and, thirdly, by large scale consistency variations in the pulp suspension supply tube associated with the headbox. In this connection, it should be noted that the initial portion of the former section of the paper machine serves to deform the pulp suspension layer discharged onto it.
If the pulp suspension flow emanating from the aperture of the headbox is constant in time but varies in dry matter content along the length of the headbox aperture, a variation in the transverse direction in the dry weight of the manufactured paper results. It is well known that such variations may be adjusted by regulating the lip of the headbox by means of fine adjustment mechanisms.
In some cases there will be no variations in the dry matter content of the pulp suspension over the length of the headbox aperture and the dry matter content of the pulp suspension will also be uniform on an average basis defined over a prolonged period of time. However, in such cases, it is not unusual for the dry matter content of the pulp suspension to differ from one moment to another. In such cases, the paper produced from the pulp suspension has heavier and lighter areas randomly dispersed over the area of the paper. Such variations are commonly termed residual variations. Such variations are caused both by the influence of turbulence vortices arising in the pulp suspension flowing out of the headbox and, also, by the non-uniform distribution, on a smaller scale, of the dry matter content of the pulp suspension.
It has been found that the magnitude or amplitude of the variations in the weight per unit area in the manufactured paper depends upon the intensity or amplitude of the perturbations generated by the forces of the disturbances which give rise to these perturbations as well as to the capability of the particular pulp suspension delivery system to attenuate, or in some cases to amplify, the perturbations having the various frequencies which either enter the system or are generated therein.
Thus, the large scale consistency variations which are characterized by low frequency perturbations can be damped or attenuated using known consistency control apparatus. Since such consistency control apparatus are of a nature that they are relatively slow to act and, furthermore, since new consistency perturbations are produced at the point where the pulp suspension is mixed prior to entering the headbox, it is desirable to provide a passive damping mechanism in the pulp suspension delivery system prior to the headbox.
An apparatus for damping such pressure and consistency perturbations has previously been developed by the applicant. This apparatus comprises an equalizing tank located in the approach pipe system, the tank providing an air cushion for the pulp suspension which is fed into and out of the tank in accordance with a plural delay principle which serves to attenuate at least the consistency perturbations and has been dimensioned in accordance with the frequency spectra of the perturbations as well as with a view toward the attenuation of such perturbations. Thus, this equalizing tank comprises a preferably horizontal cylindrical shell and cooperates with uniformly spaced input and output pipes for the pulp suspension located on opposite sides of the tank and which are arranged in accordance with the plural delay principle which is used in the prior apparatus. The input and output pipes are preferably connected to conical shaped collecting tubes which direct the inflow and outflow of the pulp suspension in mutually opposed directions.
It has been found that the operation of the above-described apparatus has not been entirely satisfactory. Thus, as mentioned above, in this previously designed damping apparatus the distribution header and the collecting header are connected by pipes provided at appropriate intervals with the equalizing tank. The structure of this apparatus, in addition to being relatively complex, has the principle drawback that where the consistency perturbations in the pulp suspension flowing through the distribution header have a wave length approximating the spacing of the header connecting pipes, it has been found that such perturbations will not be significantly attenuated.